You will be given the option to make the deposit in one lump sum or through payroll deduction. All post-56 military service deposits must be made to the Defense Finance and Accounting Service (DFAS) before separation from Federal employment. Even if you dont plan to retire or leave Federal service soon, it is a very good idea to at least obtain the amount of your military earnings so that the deposit can be computed more easily. To process the deposit, we can use your actual military pay vouchers for complete periods of military service in computing the military deposit or you can obtain a certified estimate of your military earnings from the appropriate branch of service. The DLA Human Resources Center, Customer Support Offices (CSO) can provide instructions for obtaining the certified estimate.
Once you do complete the deposit, the DFAS payroll office will send you a receipt that indicates that your deposit was paid in full, the amount paid and the period of service the deposit covers. Once you receive the receipt you will need to forward a copy to the CSO office that services you and it will be filed in your Official Personnel Folder as a permanent record. Our office needs this information from you in order to determine whether you can receive credit in your CSRS or FERS retirement benefit.
When an individual leaves Federal service or moves to another payroll offices servicing, their payroll records are closed out and sent onto the Office of Personnel Management (OPM). OPM will maintain these records until the individual applies for retirement, requests a refund or dies.
Please note that this is a brief summary of the provisions for making deposits to receive credit for post 1956 military service in a retirement benefits. Under some circumstances, particularly active military service performed after August 1, 1990 that interrupts creditable civilian service (e.g., mobilizations for Desert Shield/Desert Storm, etc.) special deposit provisions may apply due to the Uniformed Services Employment and Reemployment Rights Act of 1994 (USERRA) law.
Above Information Courtesy of Department of Defense

